System GroundingImpact on reliability and Safety

Presented by:Daleep MohlaSergio Panetta Jim ChannonScott Lee

Agenda

• System Grounding Considerations for  600 Volts and less system

• Types of System Grounding available

• Pros and Cons of each

• Mitigation strategies

• Selection guideline for mitigation 

• Questions 

Systems Grounding Considerations

• Cost?

• Safety?

• Reliability ?

• Impact of unintended outages?Production loss?Environmental?Safety?

Types of System Grounding

• Ungrounded 

• Solid Grounded

• Impedance Grounded

UNGROUNDED (DELTA) SYSTEM

L

O

A

D

Transformer Secondary

Feeder (with capacitance to ground)

Normal operation Ground fault on phase C

Neutral point establishedby distributed capacitance

• Phase C at ground potential• No fault current (no return path to source)

All phases are atline‐to‐neutral voltageabove ground (eg: 347V) •A & B phases are

at line‐line voltageabove ground(eg: 600V)

UNGROUNDED  SYSTEM:NORMAL OPERATION & FAULTED OPERATION

Normal operationIntermittent ground fault on phase C

•A & B phases are:> Line‐line voltage above ground

• Phase C > ground voltage• Intermittent fault current • Personnel danger

UNGROUNDED SYSTEM:FAULTED OPERATION WITH TRANSIENT OVERVOLTAGE

UNGROUNDED SYSTEMS:Pros and Cons

Pros

• Minimum Initial cost

•Ability to run with   one phase faulted to ground

•Isolation of Primary and Secondary currents  ( harmonics)

Initially  used by the industry to prevent  unplanned outages

Ungrounded Systems

• Cons

• Difficult to detect ground‐ faults — no fault current

• Running with a ground‐fault  increases stress on insulation, leading to phase‐to‐phase faults

• Intermittent fault may cause transient overvoltage

Wye‐Connected Transformer Secondary

Feeder (with capacitance to ground)

L

O

A

D

SOLIDLY GROUNDED SYSTEM

Solidly Grounding System

Pros

• Minimum first costs

• Immediate isolation of faulted system

• Voltage stabilization

• Allows use of neutral for single phase loads

• Visible detection of faulted equipment

• Equipment insulated rated for phase to ground voltage

Solidly grounded Systems

CONS• Unplanned outages

• Voltage dips during fault conditions

• High arcing currents through grounding systems

• Motor terminal box covers have been reported blown away 

• Fire Hazard  especially in Hazardous (Classified) Areas

• High repair cost and time 

Wye‐Connected Transformer Secondary

Feeder (with capacitance to ground)

L

O

A

D

NGR

RESISTANCE GROUNDED SYSTEM

NGRTransformer Secondary Feeder (with 

capacitance)

L

O

A

D

Current Limited to NGR Let‐Through

RESISTANCE‐GROUNDED SYSTEMWITH A GROUND FAULT

IF X

Resistance ( Impedance) Grounded Systems

Pros• Reduces unplanned outages

• Transient stability of the system 

• Eliminates undesired voltage dips during fault conditions

• Allows fault detection of the faulted equipment

• Minimizes arcing fault current and arc flash hazard

Resistance ( Impedance) Grounded Systems

• CONS

• Initial investment

• Fault detection and removal required

• System Integrity maintenance  required

• Equipment  insulation rated for phase to phase voltages

Protection or Prevention:Which is more effective?

• Seat belts, Airbags , *Accident avoidance?• *alarm and automatic  response when close to another car)

• NFPA 70E/CSA Z 462  primarily address protection of personnel to minimize personnel injury  due to electrical hazards : Shock and Arc Flash by removing power  with some soft suggestion on reduction of hazards

• Engineering controls  minimize the potential of electrical incidents

• Safety By design minimizes injury potential.• Role of High Resistance Grounding?

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Ground Faults

• There has been documentation over the years indicating that between 80% and 95% of all electrical faults initiate as ground faults.  By limiting the ground fault current to a small magnitude, a great majority of all phase‐to‐phase arcing faults can be eliminated.

• Informal numbers are that less than 1% of the faults are initially start as three phase faults (jumpers left installed, snakes etc)

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• 120.3 FPN No. 3:…high-resistance grounding of low-voltage and 5 kV (nominal) systems, … are techniques available to reduce the hazard of the system

Power System Grounding

Mitigation Electric Shock and Arc Flash Energy‐ A Total System Approach for Personnel and Equipment Protection              IEEE IAS PCIC 2010

• 4.3.1.1 Note (3)…high-resistance grounding of low-voltage and 5 kV (nominal) systems, … are techniques available to reduce the hazard of the system

Power System Grounding

Mitigation Electric Shock and Arc Flash Energy‐ A Total System Approach for Personnel and Equipment Protection              IEEE IAS PCIC 2010

70E‐ 130.5 

• Informational Note No. 3: The occurrence of arcing fault• inside an enclosure produces a variety of physical phenomena

For example, the arc energy resulting from an arc developed in the air will cause a sudden pressure increase and localized overheating.

• Equipment and design practices are available to minimize• the energy levels and the number of at‐risk procedures that• require an employee to be exposed to high level energy• sources. Proven designs such as arc‐resistant switchgear,• remote racking (insertion or removal), remote opening and• closing of switching devices, high‐resistance grounding of• low voltage and 5 kV (nominal) systems, current limitation,• and specification of covered bus within equipment are 

techniques available to reduce the hazard of the system

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High Resistance Grounding SystemsHigh‐Resistance Grounded

• There is minimal  arc flash hazard, as there is with solidly grounded systems on the first fault, since the fault current is normally limited to  a  very low value 

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High Resistance Grounding

• Does Resistance Grounding really reduce:

• Unplanned System outages?

• Potential of arc flash hazard?

Engineer’s answer  “ It depends”

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High Resistance Grounding

• Yes if and only if  

• Neutral grounding  system Integrity is maintained .AND

• first fault is cleared before inception of second fault. (Second fault on the system will result in a phase to phase fault IF the FIRST FAULT IS NOT CLEARED)

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Possible  Loss of Neutral Path

• Open or Shorted Neutral Path to Ground

ResistorFailure

LooseConnection

Broken Wire

Broken orGrounded Wire

Corrosion

Excerpt from a technical paper presented at 2009 IEEE IAS Electrical Safety Workshop

Hazard Control Measures

Hazard Control Measuresoutlined in ANSI Z10

Elimination Substitution EngineeringControls

Warnings AdministrativeControls

PPE

Addressed in NFPA 70E

Addressed in 70E Tables

Prevention Protection

An effective electrical safety program incorporates all control measures

Loss of Ground in HRG Systems 28

AØ BØ

CØ

N N

HRGCØ

BØAØ AØ BØ

CØHRG

N

HRGCØ

Open Circuit:

• Desired fault current cannot flow.

• Ungrounded System.

Open Circuit:

• Desired fault current cannot flow.

• Grounded thru high inductive transformer.

• Resonance System.

Short Circuit:

• Undesired fault current can flow.

• Place CT close to N, >costs (elevated N).

• Solidly Grounded System.

Warning of Risk

Loss of Ground in HRG Systems 29

AØ BØ

CØHRG

N

SensingResistor

Relay

• Ground Fault Relay & Sensing Resistor– Detects Open / Short Circuits and maintains Grounding

Automatic Reduction of Risk

Avoiding Second Ground Fault

• To reduce arc flash hazard, it is critical to reduce the possibility of two faults at the same time.

• How?Warning  of risk

• Either make sure maintenance people remove the ground fault immediately 

• Automatic reduction of risk• Provide sensing  equipment to prioritize feeders to avoid second simultaneous ground fault

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FAULT INDICATION USING 3 LAMPS

Indication of first Feeder fault

AN

LOAD 1 LOAD 2

ZSCTZSCT

M1 M2

GMMETER

GMMETER

FAULTED FEEDER INDICATION

- Alarm on first faultTrip on second faultCannot prioritize essential feeders Difficult to locate fault(s)

R R R

R 2.5APICKUP

100APICKUP

100APICKUP

100APICKUP

5A NGR

MGFRrelays

Minimizing  Second Simultaneous Ground Fault

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Faulted FeederFirst Fault Alarm

Faulted Phase Indication

Second Fault TripSelective Inst. Feeder TrippingGround Current as % of Let

Through

DDAI

DDR2

NGRALARM

FM1 FM2 FM3 FM4CM

DSP MKII

Combined DSA/DSPIdentifies individual faultedload and PhaseSecond Fault Protection Backup

DDR2SWGR

DDR2 DSA

ALARM

ALARM

CM FM1 FM2 FM3 FM4

CM FM1

DSP MKII

TRIPS

DSP RelayDouble ended Unit sub Application

Parallel Generators

GENERATORS 600V

600V

DDR2-6

DSA

15-20A, 3P100 kAIC

5A, 347V NeutralGrounding Resistor

G G G G

To BMS

To BMS

TYPICAL PARALLEL GENERATOR HIGH RESISTANCE GROUNDING SCHEME

Zero Sequence Current Sensors(one per feeder; one per generator)

OptionalPulsing Resistor

5A, 600VZig-Zag GroundingTransformer

2 - #16AWG, 24 Vdcfor pulsing control

See Notes 1 and 2.

Notes:1. NGR/Zig-Zag assembly w ith pulsing resistor, IPC Part Number: OHMNI-6PM-5-ZZ2. NGR/Zig-Zag assembly w ithout pulsing resistor, IPC Part Number: NTR600-5-ZZ

DS-P

M2

Optional DS-PM2 Pulsing Card

AWG#8 as perCEC 10-1108(3)

Mitigation Strategies

• Good System maintenance practices

• Better  ‐WarningsAlarms Procedures PPE

• Best ‐Automatic Reduction of RiskEliminationEngineering Controls

Mitigation Strategies selection

• Which one to use?

• “ It Depends”

• Consider  safety/cost/benefit 

• Impact  due to failure in the systemCan system be shut down immediately?Cost of shutdown?

• Maintenance practices?

• Environmental conditions?

• Liability considerations of design?

Risk reduction

• Key strategy in 70E/ CSA Z 462  is  risk analysis

• Risk = Frequency X Consequences

• Use of HRG reduces the probability of frequency and thus reduces the risk 

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Risk Analysis

• Does HRG reduces the incident energy required to be put on label?

NO. Label is based on 3 phase bolted fault

BUT• it does reduce probability of high incident energy (80‐ 95%) of the time and can be utilized in Risk Analysis.

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Questions ?

Thank you for being here !!

• Thanks for providing me with a forum for preaching electrical safety by design 

• Daleep Mohla ,IEEE Fellow , P.E

• d.c.mohla@ieee.org

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